In a relatively large fuel system having more than one individual fuel tank for providing fuel to one or more engines, such as an aircraft fuel system, it is common practice to transfer fuel from one fuel tank to one or more other fuel tanks whilst the fuel system is in use. In an aircraft fuel system this is done not only to ensure that each of the aircraft engines has fuel available at all times but to also ensure that the changing weight distribution of the aircraft as the fuel is consumed is maintained within certain limits
In an aircraft fuel system the transfer of fuel between individual fuel tanks is ordinarily achieved by pumping the fuel from a source tank to one or more receiving tanks by opening and closing one or more fuel valves located within the fuel system. By providing one or more sensors on the fuel pumps and/or the valves it is possible to detect certain failure conditions within the fuel system, for example sensors can be provided to determine if a valve is in an open or shut condition and pressure sensors can be provided to determine if a fuel pump is operational or not.
However, there remain certain equipment failures or conditions that are not easily detectable and that cause a failure in the desired fuel transfer. For example, a valve shaft failure or blockage in the fuel transfer system are not presently easily detectable but can lead to failure of the fuel transfer.
In the event of the failure of a pumped fuel transfer on an aircraft it is normal operating practice to attempt a gravity fuel transfer in which it is attempted to transfer fuel from a source fuel tank to one or more receiving fuel tanks, where the source fuel tank is orientated above the receiving tanks within the fuel system at that time. For example, in some aircraft in addition to the fuel tanks located within the main aircraft wings an additional fuel tank, often termed the trim tank, is located within the horizontal stabilizers at the rear of the aircraft. Under normal flying conditions the trim tank is above at least some of the wing tanks. Consequently, provided the aircraft is in a suitable attitude then fuel can be fed under the effect of gravity from the trim tank to one or more of the wing tanks. Additionally, the outer wing tanks of an aircraft are often situated above the inner wing tanks when the aircraft is in a level flight attitude and consequently fuel can be transferred by gravity from the outer tanks to the inner wing tanks.
Whilst the aircraft flight crew generally have one or more instrument displays available to them informing them of the quantity of fuel held within any one of the individual fuel tanks, the rate at which the fuel quantity will change, i.e. the flow rate, during a fuel transfer may be relatively low, for example of the order of 3 to 20 tonnes an hour for a gravity transfer or 15 to 35 tonnes an hour for a pumped transfer. Consequently, even if the fuel transfer is progressing normally the flight crew are required to monitor the fuel quantity indicators over a relatively prolonged period of time to determine if the fuel transfer is proceeding at the expected rate. Often the flight crew need to review and monitor other instruments at the same time and it is therefore quite plausible that the fuel transfer is not monitored correctly by the flight crew. Additionally, the operation of the fuel system on the aircraft is typically arranged such that a fuel transfer may be instigated automatically by the aircraft control systems, in which case the flight crew might not be aware of the need to monitor the fuel transfer rate.